When stamping or forming parts with a press, the formed or coined dimensions of the parts are dependent on the shut height of the press remaining constant. To ensure that the formed or coined dimensions of the parts are acceptable, it is desired that the shut height of the press remain constant or be within a certain tolerance dictated by the nature of the parts. Some precision forming or coining operations, such as those relating to the manufacture of electrical connectors, require that the shut height of the press be held to plus or minus 0.005 mm. If the shut height does not remain constant, part dimensions will vary, possibly outside of allowable tolerances.
Due to thermal and other factors, shut height normally varies slowly as the press runs at constant speed. As the press is stopping and starting, however, the press is operating at a lower speed than the constant speed the press attains during steady state operation and the shut height varies. This shut height variation during starting and stopping is due mainly to dynamic forces.
The typical shut height variation with press speed in strokes per minute (spm) as a high speed press starts up is depicted in FIG. 1. As depicted in FIG. 2, for the operation of a typical press during a particular run, the shut height drops from its initial value at start up at time A to a steady state value within a desired shut height range at time B. From time A to time B, the press speed rises from zero (0) to some relatively constant steady state press speed. During steady state operation from time B to time C, the shut height and press speed remain relatively constant until the press begins to stop at time C. From the initiation of the stopping of the press at time C until the time the press is stopped at time D, the shut height increases while the press speed decrease to zero (0).
The variation in shut height while a press is starting and stopping, as depicted in FIG. 2, means that part dimensions will vary more, possibly out of tolerance values for the parts, as the press is starting and stopping between time A and B and between time C and D.
Shut height variation occurs in presses operating at all speeds. However, shut height variation is much more pronounced, particularly while the press is starting and stopping, for presses operating at higher press speeds (generally above 600 spm).
Technology currently exists for maintaining a constant shut height as a press is running at constant, steady state operational speed. Until the present invention, no technology has existed for compensating for the variation in shut height as a press stops and starts and the press speed is varying. Current technology requires that material be fed to the press while the press is starting and stopping and the press shut height is varying, thereby producing bad, out of tolerance parts due to the varying shut height. The present invention eliminates the production of bad, out of tolerance parts due to varying shut heights as the press is starting and stopping.